FIG. 3 shows a circuit according to a conventional system which is disclosed in Patent Document 1 (which is identified below). In FIG. 3, reference numeral 1 denotes an AC power supply, reference numeral 2 denotes a inductor, reference numerals 3 to 6 denote diodes, and reference numerals 7 to 10 denote semiconductor switches, which are taken here to be MOSFETs (Metal-oxide-semiconductor field effect transistors). The MOSFETs are always in a conducting state in the reverse direction, due to the presence of a parasitic diode (illustrated). By connecting the MOSFETs in a reverse series arrangement, a so-called bi-directional semiconductor switch is composed which is capable of controlling the passing and shutting off of current in both the forward and reverse directions. Reference numerals 11 and 12 denote capacitors, and reference numeral 13 denotes a load. Here, there may also be cases where a load is connected in parallel with the capacitor 11 or 12.
This circuit is a so-called rectifier circuit which converts alternating current to direct current, and has a function for keeping the DC output (P-N) voltage Vout to a desired value higher than the peak value of the AC input voltage Vin, while keeping the input current Iin to a sinusoidal waveform of equal phase to the AC input voltage Vin.
The operation for achieving these functions will be described below. For example, if the AC input voltage Vin is positive and the voltage relationship is Vin<Vout/2, then when the gates of the bi-directional semiconductor switch MOSFETs 7 to 10 are all on, the potential of the contact point U1 of the diodes 3 and 5 is equal to the potential of the point M of the DC output and the point V of the AC power supply 1, and the voltage between U1 and V becomes 0 V. Therefore, a current flows along a path from the AC power supply 1, to the inductor 2, to the MOSFET 7, to the MOSFET 8, to the MOSFET 10, to the MOSFET 9, to the AC power supply 1, and the voltage Vin of the AC power supply 1 is applied to both ends of the inductor 2 and the input current Iin increases. This state is called a 0 voltage mode below.
Next, when the MOSFET 7 is switched off, a current flows along a path from the AC power supply 1, to the inductor 2, to the diode 3, to the capacitor 11, to the MOSFET 10, to the MOSFET 9, to the AC power supply 1, thereby charging the capacitor 11. In this case, the differential voltage between the AC input voltage Vin and the voltage Vout/2 of the capacitor 11 is applied to the inductor 2, and the input current Iin is reduced. The voltage between U1 and V in this case is the voltage Vout/2 of the capacitor 11. Below, this state is called a 1/2 voltage mode 1a. 
Furthermore, when the MOSFET 10, rather than the MOSFET 7, is switched off, a current flows along a path from the AC power supply 1, to the inductor 2, to the MOSFET 7, to the MOSFET 8, to the capacitor 12, to the diode 6, to the AC power supply 1, thereby charging the capacitor 12. In this case, the differential voltage between the AC input voltage Vin and the voltage Vout/2 of the capacitor 12 is applied to the inductor 2, and the input current Iin is reduced. The voltage between U1 and V in this case is the voltage Vout/2 of the capacitor C12. Below, this state is called a 1/2 voltage mode 1a. 
If the AC input voltage Vin is positive and the voltage relationship is Vin>Vout/2, then when the gates of MOSFETs 7 and 8 are off and the gates of the MOSFETs 9 and 10 are on, current flows along a path from the AC power supply 1, to the inductor 2, to the diode 3, to the capacitor 11, to the MOSFET 10, to the MOSFET 9, to the AC power supply 1, thereby charging the capacitor 11. In this case, the voltage differential between the AC input voltage Vin and the voltage Vout/2 of the capacitor 12 is applied to the inductor 2, and in this case, since the AC input voltage Vin is greater than the voltage Vout/2 of the capacitor 11, then the input current Iin increases. The voltage between U1 and V in this case is the voltage Vout/2 of the capacitor 11. Below, this state is called a 1/2 voltage mode 1b. 
Similarly, when the gates of the MOSFETs 7 and 8 are switched on, and the gates of the MOSFETs 9 and 10 are off, a current flows along a path from the AC power supply 1, to the inductor 2, to the MOSFET 7, to the MOSFET 8, to the capacitor 12, to the diode 6, to the AC power supply 1, thereby charging the capacitor 12, and the input current Iin increases. The voltage between U1 and V in this case is the voltage Vout/2 of the capacitor 12. Below, this state is called a 1/2 voltage mode 2b. 
In other words, in the name of the 1/2 voltage mode indicated above, the suffix 1 represents a mode of charging the capacitor 11, the suffix 2 represents a mode of charging the capacitor 12, the suffix a represents a mode wherein the input current Iin decreases, and the suffix b represents a mode wherein the input current Iin increases.
From these respective modes, if the gates of the MOSFETs 7 to 10 are all switched off, then a current flows along a path from the AC power supply 1, to the inductor 2, to the diode 3, to the capacitor 11, to the capacitor 12, to the diode 6, to the AC power supply 1, and the differential voltage between the AC input voltage Vin and the DC output voltage Vout is supplied to the inductor 2. In a normal operational state of the device, the AC input voltage Vin is lower than the DC output voltage Vout, and the input current Iin decreases. The voltage between U1 and V in this case is the DC output voltage Vout. This state is called a full voltage mode below.
By switching the MOSFETs 7 and 8, or the MOSFETs 9 and 10, at high frequency, and controlling the time ratio of this on/off switching, the shape and size of the input current Iin can be controlled arbitrarily if the mode described above is switched. Consequently, the waveform of the input current Iin is taken to have a sinusoidal shape (here, ripples are ignored), and furthermore, it is possible to keep the DC output voltage Vout at a desired value by controlling the amplitude of the input current Iin in accordance with the load power. The same operation is possible when the AC input voltage Vin is negative. As indicated above, this circuit is a so-called three-level circuit, which is capable of holding the voltage between U1 and V at three voltages: 0 V; Vout/2; and Vout.
In the operation described above, if the input current Iin is positive and the MOSFETs 7 and 8 are switched on, for example, then provided that the gate of the MOSFET 7 is on, the MOSFET 8 will be in a conductive state due to the parasitic diode, regardless of whether the gate of the MOSFET 8 is on or off, but a current also flows in the MOSFET 8 itself due to the on signal being applied to the gate of the MOSFET 8. The diode has a forward voltage drop of no less than a fixed value, regardless of the current, but since the forward voltage drop of the MOSFET has the characteristic of being directly proportional to the current, then it is possible to reduce the forward voltage drop especially when the current is small, which is effective in reducing conduction loss. This is well known generally as synchronous rectification technology. Consequently, control is implemented to apply an on signal to the gates of all of the MOSFETs which are to pass current, regardless of the polarity of the current, as described above.
Patent Document 1: Japanese Patent Application Publication No. 2008-22625